This is a rather weird-looking exercise, but it's a very effective way to build the whole deltoid with an emphasis on the lateral portion.
Basically, the movement is like performing a one-arm lateral raise while the other arm is brought up with the elbow bent at a 90 degree angle. The bent arm doesn't do much work because of the advantageous lever, but it allows you to keep some tension on the shoulder muscle during the whole exercise. Constant tension equals more growth!
It seems that in every family there's always a black sheep, someone who just doesn't quite fit in with the rest. It might be your crazy Aunt Sally who's run off and joined a cult, or maybe your family has its own cousin Eddie of National Lampoon "Vacation" fame.